A signal acquired from a sensor mounted on a vehicle usually includes a drift component, and the drift component sometimes exceeds a normal state temporarily because of environmental changes such as temperature. As for the effect of the drift component, when calculating the angle of the vehicle by integrating the output from an angular velocity sensor, there are some cases where the drift component is also integrated and hence causes a considerable error with respect to a real angle. Accordingly, it becomes necessary to eliminate the drift component from the angular velocity sensor as preprocessing of the integral.
Conventionally, to solve the foregoing problem, a drift correcting method has been known which extracts the drift component of the sensor signal from a vibration gyro or the like using a high frequency elimination filter, and eliminates it by subtracting from the sensor signal (see Patent Document 1, for example).    Patent Document 1: Japanese Patent Laid-Open No. 7-27774/1995.
According to the technique disclosed in Patent Document 1, although the drift component is eliminated, a subtrahend is small in the high frequency elimination filter set for the drift component from the sensor in a steady state. Accordingly, it has a problem of taking a considerable time to eliminate the drift component temporarily occurred because of the environmental changes.
Therefore it is necessary to increase the cutoff frequency and order of the high frequency elimination filter so as to increase the subtrahend up to such a level that enables suppression of the drift component.
However, setting the cutoff frequency and order at high offers a problem of bringing about an overshoot phenomenon in which the subtrahend becomes excessive with respect to the sensor output in the latter half of the output signal for a target physical phenomenon like a rollover.
In a starting device for activating an occupant protective system of the vehicle, when an overshoot is large, an overshoot component occurs in a right turn with respect to a rollover phenomenon of a left turn vehicle, for example. In this case, there is a possibility of erroneously starting the occupant protective system mounted on the vehicle for the right turn.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a starting device of an occupant protective system capable of eliminating a drift component quickly for the drift component occurring temporarily owing to environmental changes, and capable of achieving offset elimination in such a manner as to prevent the overshoot in the latter half of the output signal for the target physical phenomenon.